Multiple focal length eyeglasses of various types are well-known in the art. The most familiar are the bifocal lens variety wherein the upper portion of each optical lens consists of a longer focal length prism, suitable for aiding an individual's distance vision, and the lower portion of each optical lens consists of a shorter focal length prism, suitable for aiding an individual's reading vision. Such bifocal type eyeglasses suffer, however, from many objectionable features. Many persons consider them unsightly and embarrassing to wear because the horizontal line along which the upper and lower lenses are joined is normally readily visible, and such lenses are frequently associated with elderly persons even though many middle-aged persons require separate eyeglasses for reading and for normal wear. Furthermore, an individual's field of vision is significantly restricted by bifocal lenses. The upper portion of such lenses is useful only for distance vision, the lower portion of such lenses is useful only for close or reading vision, and the center portion of such lenses is partially obscured by the dividing line between the upper and lower portions.
Accordingly, for a long time, efforts have been made to obtain the convenience of variable focal length properties in a single pair of eyeglasses while eliminating the objectionable features of bifocal lenses. To this end, several varieties of adjustable focal length optical lenses have been proposed. One variety of adjustable focal length optical lenses consists of a non-flexible lens having a partially liquid-filled cavity as shown in U.S. Pat. No. 4,174,156 (Glorieux). When the axis of this lens is horizontal, the central portion of the cavity contains only air; when the axis of this lens is forwardly inclined, the center portion of the cavity fills with liquid thereby changing the focal length of the lens depending on the liquid used and the size of the cavity. It should be apparent that variable focal length lenses of this type suffer from exactly the same objectionable features as do more conventional bifocal lenses.
A variation of this concept is shown in U.S. Pat. No. 2,437,642 (Henroteau) which eliminates at least one of the problems of the Glorieux patent. In Henroteau, the liquid is stored in small reservoirs attached to the lens frames and is, alternately, added to or removed from the lens' cavities as required by rotating the lens frames. Thus, this invention eliminates the unsightly line of demarcation across the face of the lenses and expands the field of vision. On the other hand, it requires the presence of, perhaps, even less aesthetically appealing liquid reservoirs attached to each lens frame.
A second variety of adjustable focal length optical lenses consists of flexible lenses of several types and used in different ways. One type of variable focus eyeglasses using flexible lenses is shown in U.S. Pat. Nos. 3,598,479 (Wright) and 4,181,408 (Senders). These patents describe eyeglasses employing at least partly flexible liquid-filled lenses in which the curvature of the lens, thus its focal length, can be varied by adding fluid to or withdrawing it from an interior cavity in the lens. In these patents, the liquid reservoir is cleverly concealed in the temple bow or side arm portions of the eyeglass frames. Each of these patents, however, requires a delicate, complex and relatively expensive mechanical or electromechanical mechanism in order to pump the working fluid into and out of the flexible lenses as required. Also, with the eyeglasses in constant use, there is the ever-present possibility of rupturing the small ducts which carry the fluid between the lenses and the reservoirs. This is particularly true at the corners where the side arm portions of the eyeglass frame are movably joined to the front or lens-holding portion of the frame. It is perhaps for this very reason that the Senders patent has the side arms immovably joined to the front portion of the eyeglass frame.
Still another variation of the use of flexible lenses is where the curvature, thus the focal length, is varied by changing the shape of the lens frame. Such embodiments are briefly disclosed in U.S. Pat. No. 2,300,251 (Flint) and British Pat. No. 758,668 (Russell) although neither of these patents suggests a specific means of accomplishing this result.
In short, none of the prior art teaches an entirely satisfactory mechanism for varying the focal length of optical lenses.